Power balancing in autonomous
renewable energy systems
The design and construction of the renewable energy laboratory
DENlab ®
Power balancing in autonomous
renewable energy systems
The design and construction of a renewable energy laboratory
DENlab ®
PROEFSCHRIFT
Ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus Prof. dr. ir. J.T. Fokkema, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op maandag 24 november 2008 om 12:30 uur door
Arjan Marco van VOORDEN
elektrotechnisch ingenieur geboren te Rotterdam
Dit proefschrift is goedgekeurd door de promotor:
Prof. ir. L. van der Sluis
Copromotor:
Dr. ir. G.C. Paap
Samenstelling promotiecommissie:
Rector Magnificus, voorzitter
Prof. ir. L. van der Sluis, Technische Universiteit Delft, promotor Dr. ir. G.C. Paap, Technische Universiteit Delft, copromotor Prof. dr. ir. J. Hellendoorn, Technische Universiteit Delft Prof. ir. W.L. Kling, Technische Universiteit Delft Prof. dr. J. Schoonman, Technische Universiteit Delft
Prof. dr. ir. J. Driessen, Katholieke Universiteit Leuven, België Dr. ir. F. van Overbeeke, EMforce B.V.
Dit onderzoek is financieel ondersteund door het Samenwerkingsverband Duurzame Energie (SDE) en het onderzoeksprogramma Sustainable ENergy: Extraction, Conversion and Use (SENECU), TU Delft.
Printed by: Drukkerij Verloop, Alblasserdam.
ISBN:
Copyright © 2008 by Arjan van Voorden
All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without permission form the publisher or author.
PREFACE
A thesis like this one is the result of the efforts of not only one person. Some efforts contribute directly to the thesis by giving guidance during the research, comments on the manuscript or assist during the laboratory set-up and its measurements. Other people contribute more indirectly to the thesis, but provide in other ways support during the research and dissemination.
Firstly, I want to thank my colleague ing. Johan Vijftigschild for his many contributions during the research, especially for practical assistance, but not limited to that only. We had many discussions about how to program the PLC software and how to adept the standard Siemens components to be useful for our purpose. Together with the project leader, Dr. ir. Bob Paap, we worked as the DENlab team.
Dr. Paap is the one I will thank secondly for his initial ideas and support at the difficult moments during the installation and the monitoring phase. Beside of these activities, also thank for revising the papers and taking part in the committee.
I also want to thank my promotor, Prof. ir. Lou van der Sluis, for giving me the opportunity to perform a quite practical Ph.D-project. Ph.D. theses with a high practical component in it are not very common in our research group. One of your initial ideas was to attract and interest more students in renewable energies. We can state that this goal is realised. From primary school to Master of Science students from all over the world.
I do not forget the assistance of ing. Rob Schoevaars. The Renewable Energy Laboratory is constructed with a lot of power electronic components. Rob was always willing to help by all kind of technical problems, sharing his wide knowledge on electronic components and circuits from kilowatts to microwatts. I will thank Rob for all his time for teaching me and Johan.
The partners in the SDE (Samenwerkingsverband Duurzame Energie) project, under supervision of ir. Sigrid Bestebroer and Dr. ir. Frank van Overbeeke, ir. Rob van Gerwen, drs. ing. Koen Kok, drs. Jan Uitzinger and drs. Harm Jeeninga.
The partners in the SENECU (Sustainable ENergy: Extraction, Conversion and Use) interfacultary research project, under leadership of Prof. dr. Joop Schoonman. It was great to have discussions with colleagues which are used to milliwatts instead of our kilowatts-scale. We all try to contribute to the transition to a new clean and environmental-friendly society.
Special thanks also to my students, which I guided during my Ph.D-study. First of all my Portuguese Erasmus students, with their beautiful names: Diogo Nuno Pita da Silva e Vasconcelos (2000), Luis Miguel de Crasto Natario (2004) and Pedro Miguel Paulino (2006), but also Gael Marchand from Toulouse, France (2004) and Wilco Boonstra (2001). With their Master theses, they have made a contribution to this thesis. It was very stimulating to guide you with the different assignments.
However there was no direct contribution from my current employer, I want to thank STEDIN for the support in completing this thesis and financial support publishing in color. From a different order is my gratitude to my family. My wife, Ina, our children, Joanne, Joost and Mark, my parents who stimulate me all to continue my study after finishing the B.Sc.-study in 1995. We all did not expect that it all would lead to a Ph.D.-degree in Electrical Engineering. We all believe that this was not possibly without the gifts all people
get from their Creator. He gives me this opportunity to contribute to the society with this thesis work and with other, completely different, contributions especially under the next generation. For of Him, and through Him, and to Him, are all things: to Whom be glory for
ever. Amen. [Romans 11:36]
Arjan van Voorden
Future needs and possibilities in power supply require solutions to accommodate the changes in the world of power systems, market operation and the need to reduce greenhouse gasses. To develop scenarios for the future, is therefore essential. The desirability of reducing greenhouse gasses evokes research on the most effective application of renewable energy. This research aims at finding new ways to generate electricity from renewable sources. Besides the search for new sources and innovative conversion methods, the electrical infrastructure of the next generation also requires special attention. In both contexts, this research attempts study the autonomous electricity systems with a high penetration of renewable energy sources.
Problems with applying many renewable sources, such as wind and solar conversion systems, are the fluctuation and the difficulty in forecasting the power generation of these sources. An additional problem is the fluctuation not only in supply, but also in demand. As in every power system, the total of generated and demanded power requires balance at every moment. Using a grid connected system, inequity between demanded and generated power leads to a change in actual kinetic energy. This change in turn causes a frequency change in the network. By applying a primary and secondary control, the synchronous generators in the network react on this frequency deviation by increasing their active power. Contrary to the grid connected system, an autonomous system, with no directly coupled synchronous machines, cannot apply such a balance control. The voltage and frequency in these autonomous networks are dictated by a power electronic component, with a Voltage Source Converter (VSC) control. This type of control prevents the frequency from changing, provided the component operates within its boundaries (maximum current and operating between the DC voltage limits).
Consequently, the discrepancy between supply and demand leads to a change in actual VSC-current, in order to compensate this. A storage system makes the power that is generated, or delivered by this VSC available.
As a consequence of the fluctuations in the renewable sources, the peaks in the load demand (by considering a limited number of households) and the absence of coherence between the fluctuations, the power exchange between the storage system and the rest of the system will fluctuated in a high extent.
This means an unpredictability of the power exchange in direction but also in magnitude. (Furthermore, this power is independent of the actual state of charge of the battery storage.)
In 2000, the department Power Systems Laboratory of the Delft University of Technology decided to build a renewable energy laboratory to study the system integration of renewable energy sources in an autonomous system so as to create vision on the fluctuations in an autonomous renewable system and to design control strategies for mastering the expected fluctuation. From the onset, the size of the laboratory had to be large enough to ensure reliable predictions of larger systems (extendable results). The decision fell on a scale of ten households.
During the study on the fluctuation and the power exchange in the system, two aspects play a major role: the short term (fluctuation, short term stability) and the long term (availability of energy, long term stability).
To get insight into the power exchange and the availability of energy from the sources, the KNMI provided data on wind and Satellight on sun-irradiance data. Based on these databases with mean hour values, a first estimation was made on the potential energy and powers.
Based on the energy calculation with these data and the available space on the roofs of ten houses, it is shown at 120 m2 of solar panels, a wind turbine of 30 kW and a Combined Heat and Power set of 5.5 kWe (12 kWth) are necessary to supply the energy demand of ten houses. The size of the energy buffer is calculated to be 100 kWh, assuming conventional lead-acid batteries.
In the fist design, the laboratory was equipped with state of the art components. Together with Siemens NL, we decided to compose the laboratory with components from the driving industry. These components were then programmed with the right parameters, to obtain desired functionality. However, due to limitations inherent to laboratory set-ups, additional components were required to make ends meet. Although these additions are not necessary in the real situation, they do not influence the research activities. With all these components, the laboratory has now obtained the flexibility which is essential for a study object.
While installing all components, the system is gaining in functionality; the overall control strategies are programmed in the Programmable Logic Controller (PLC); the dedicated software, Step7, is installed; and a Profibus-based user interface is made to control, log and monitor the system behaviour. Also, all relevant units (powers, voltages, currents, SoC‟s) are stored for analyses purposes.
By the design of the optimal control strategy, two aspects are especially important: the availability of energy and the minimal use of the CHP-unit. The first concept provides a control strategy, which gives the system a high flexibility, by only limiting the maximal and minimal SoC value. The second concept studies the application of two parallel battery stacks. Here the control strategy can decide to use one or both batteries. The third strategy is to use load forecasting. Once you can forecast periods with a high risk on energy shortage, the CHP is then started up earlier to prevent low SoC periods. The fourth strategy that is examined applies equalisation processes. The final strategy forecasts the renewable energy supply, in order to minimize the use of the CHP (gas consumption). This last strategy gives the best results: a higher mean value of SoC, a lower number of CHP-starts and the lowest use of the CHP-unit, which ensures a longer battery life.
Besides these strategies, the added value of forecasting of renewable energy is investigated. This value is measured by the reduction of fossil fuel (use of CHP-unit) and the optimal use of the battery (extending lifetime). It is shown in this research that a “simple” persistence method is relative good, but the expected efficiency improvement is not realised in this system configuration.
The first results lead to believe that the system is able to reliably supply energy to the consumers. However, a closer analysis of the measurement and simulation results prove that the power fluctuations are so high and fast that an unacceptable reduction of the battery‟s life time can be expected and that its performance is influencing the energy calculations.
As a solution to this problem, the battery must be relieved by another fast and efficient storage system, such as a super capacitor. They have as an additional advantage the capability to catch the fast fluctuations of the power. In this way, the battery provides the mean value of the storage system on in a certain time frame/on any given time/in a
predetermined time frame. The remaining fluctuation in the battery power depends directly on the size of the super capacitors. The end of this thesis shows simulations with super capacitors and batteries. Two different combinations are compared: the DC and the AC coupled variant. The AC option is preferred over the DC version, because of the measure of controllability.
In conclusion, a renewable energy laboratory is realised to analyse the autonomous application of renewable energy sources to a system of ten households (50 kVA). With some small adaptations, a weak-grid, grid-connected or a parallel operating situation can be created. This flexibility, which was one of the starting-points by the set-up, gives the opportunity to also integrate other renewable sources with ease, both in hardware and in software models.
Based on the research on autonomous renewable energy systems, the autonomous operation is possible, however difficult because of the limited number of households.
Onder invloed van veranderingen in de elektriciteitswereld, aangedreven door enerzijds marktwerking en anderzijds de noodzakelijkheid om de uitstoot van broeikasgassen te beperken, wordt er gezocht naar scenario‟s waarin in de toekomst energie duurzaam geleverd kan worden aan de gebruikers. Beperking van de broeikasgassen leidt tot het onderzoek naar de meest effectieve toepassing van duurzame energie. Continu wordt er door verschillende instellingen onderzoek gedaan naar nieuwe vormen van duurzame energie afkomstig uit bronnen die bij conversie naar elektriciteit geen of een minimale bijdrage leveren aan de uitstoot van milieu belastende gassen. Naast het onderzoek naar bronnen en conversiemethoden focust het onderzoek zich ook op de elektrische infrastructuur die voldoet aan de eisen van de toekomst. In dat kader valt deze studie: het onderzoek naar autonome elektriciteitssystemen met een hoge penetratie van duurzame energiebronnen.
Problemen bij de toepassing van veel duurzame bronnen, zoals wind energie en zonne-energie is de fluctuerende en moeilijk voorspelbare opbrengst van elektriciteit. Bijkomend probleem bij een autonome toepassing met een beperkte omvang (geografisch én schaalgrootte) is de fluctuaties aan de gebruikerszijde, de elektrische belastingen.
Zoals in elk elektriciteitssysteem zal ook in een autonoom systeem de totale hoeveelheid geleverd vermogen en gevraagd vermogen op elk moment met elkaar in evenwicht moeten zijn. Bij een netgekoppeld systeem leidt een onbalans tussen vraag en aanbod tot een verhoging van de frequentie, veroorzaakt door een verandering van de kinetische energie van de synchrone machines. De verandering van de frequentie leidt via de geïnstalleerde primaire regeling op de generatoren tot een verandering van het opgewekte vermogen.
Bij een autonoom systeem waarbij mogelijk geen direct gekoppelde synchrone machine is verbonden met het netwerk, kan de balansregeling zoals bij de netgekoppelde situatie niet toegepast worden. De spanning en frequentie wordt in een dergelijk systeem bepaald door een vermogenselektronische component die in Spanningsbron regeling („Voltage Source Converter [VSC]‟) is ingesteld. Bij de toepassing van deze regeling kán de frequentie van het netwerk niet wijzigen, zolang de component functioneert binnen de grenzen (maximale stromen en voldoende gelijkspanning).
Het gevolg van de toepassing van deze regeling is, dat de onbalans tussen vraag en aanbod van vermogens leidt tot een verandering van de actuele stroom van deze VSC, waardoor de totale vermogensbalans altijd in evenwicht is. Het vermogen dat door deze stroom veroorzaakt wordt, wordt opgeslagen of komt vrij uit een opslagsysteem.
Gezien de fluctuaties in de duurzame opwekeenheden door de aard van hun primaire energiebron (zon en wind) én de niet-constantheid van de belastingsvraag én het afwezig zijn van coherentie tussen de fluctuaties, zal de vermogensuitwisseling tussen het opslagsysteem en de rest van het systeem eveneens sterk fluctueren. Dit betekent een
onvoorspelbaar gedrag in snelheid van de verandering, maar ook een
onvoorspelbaarheid van de grootte van dit vermogen. Daarbij is dit vermogen ook onafhankelijk van de momentane ladingstoestand van het opslagsysteem.
Om de systeemintegratie te onderzoeken ten einde inzicht te krijgen op de invloed van fluctuaties op het autonome systeemgedrag en regelingen te ontwerpen om deze verwachte fluctuaties beheersbaar te maken, is in 2000 besloten tot de bouw van een
duurzaam energie laboratorium. Uitgangspunt bij het ontwerp van dit laboratorium is geweest dat er een systeem moest komen met een schaalgrootte waarbij, op grond van de resultaten uit dit project, uitspraken gedaan konden worden over grotere systemen, met andere woorden de resultaten moesten extrapoleerbaar zijn. Besloten is om een schaalgrootte te kiezen van 10 huishoudens.
Bij het onderzoek naar de fluctuaties en het verloop van de energiestromen in het systeem spelen twee aspecten: de korte termijn (fluctuerende vermogens) en de langere termijn (beschikbare energie).
Om beide inzichten te verwerven is gebruik gemaakt van beschikbare meetdata van o.a. het KNMI (windgegevens) en de Europese database Satellight (instralinggegevens). Op basis van deze gegevens is een eerste inschatting gemaakt van de fluctuaties die voor kunnen komen in een systeem met deze energiebronnen.
Uit de energieberekeningen op basis van deze gegevens en de beschikbare ruimte op standaard huizen, is gebleken dat als bronnen 120 m2 zonnepanelen nodig waren (12 m2 per huis), een wind turbine van 30 kW en een micro warmte krachteenheid van 5.5 kWe. De grootte van de energiebuffer moest 100 kWh bedragen, uitgaande van conventionele lood-zuuraccu‟s.
Bij de bouw van het laboratorium is geprobeerd uitsluitend met „state-of-the-art‟ componenten te werken. In samenspraak met Siemens Nederland is gekozen voor een opstelling met componenten die ontworpen zijn voor de aandrijfwereld. Door de componenten met de juiste instellingen te programmeren is de gewenste functionaliteit gerealiseerd. In de huidige opstelling zijn er een beperkt aantal operationele beperkingen en een aantal componenten, dat in een echte realisatie van het systeem niet geïnstalleerd zullen worden (b.v. transformatoren voor galvanische scheiding). Niettemin is een systeem opgezet waarmee het gewenste onderzoek kan plaatsvinden en dat voldoende flexibiliteit bezit om het huidige, maar ook het toekomstige onderzoek uit te voeren.
Bij de installatie en inregeling van alle componenten is de gewenste functionaliteit aan het systeem gegeven. Hierbij behoort ook de externe hardware (zonnepanelen en omvormers) en de signaalgevers (belastingspatroon en anemometer). Deze regelingen zijn geprogrammeerd in de dedicated controller van het systeem een PLC (programmable logic controller). Hiervoor is gebruik gemaakt van de dedicated software (Step 7) voor het programmeren van een PLC. Hiermee kan het systeem volledig zelfstandig zijn beslissingen nemen. Daarnaast is met behulp van een eigen geschreven software programma een interface gemaakt op basis van het Profibus systeem, waarbij de controle van het systeem ook handmatig uitgevoerd kan worden. Bij dit software programma hoort ook een dataopslag systeem van alle relevante grootheden van het systeem (vermogens, spanningen, stromen en ladingstoestanden). Op basis van deze meetgegevens wordt de werking van het systeem ook verder geanalyseerd.
Bij het ontwerpen van een optimale regeling wordt er enerzijds gekeken naar de beschikbaarheid van energie en anderzijds naar het minimale gebruik van de warmtekrachteenheid. Als eerste regelconcept is gekozen voor een regeling waarin een grote mate van vrijheid aan het systeem gegeven wordt. Hierbij wordt alleen grenzen gesteld aan de maximale en minimale ladingstoestand van de batterij (95% resp. 20%). Als tweede concept is de toepassing van twee parallelle batterij stacks onderzocht, waarbij de regeling kan beslissen om bij grote stromen deze te verdelen over twee batterijen en om in andere periode de hoogst opgeladen batterij te ontladen en de minst geladen batterij op te laden.
Als derde strategie is de toepassing van belastingsvoorspelling onderzocht. Indien er periode voorzien worden waarin er een tekort dreigt te ontstaan, kan in een eerdere fase de warmtekrachteenheid ingeschakeld worden om diepe ontladingen te voorkomen. De vierde strategie die onderzocht is, is de toepassing van processen om de verschillen in de ladingstoestand van in seriegeschakelde batterijen te vereffenen (“equalization proces”). Als laatste is naast de voorspelling van de belasting, ook het energie aanbod voorspeld om de regeling te optimaliseren. Optimaal gedrag is dan het minimale gebruik van de warmtekrachteenheid (gasverbruik) met een goede omgang met de batterijen eenheid.
Daarnaast is onderzocht in hoeverre aanbod voorspellers (zonne- en windenergie) een bijdrage kan leveren aan een betere prestatie van het energiesysteem. Deze prestatie wordt gemeten in een beperking van het gebruik van de warmtekrachteenheid (gebruik van gas) en een optimaal gebruik van de batterij (levensduur verlenging). Uit het onderzoek is gebleken dat een eenvoudige “persistentie” voorspeller redelijk voldoet, al blijkt de meerwaarde ten opzichte van de systemen zonder aanbodvoorspeller relatief te zijn, voor deze systeemconfiguratie.
Uit de eerste resultaten is gebleken dat het systeem inderdaad in staat is om een betrouwbare levering van elektriciteit te waarborgen. Echter, uit nadere analyse van de meetresultaten is gebleken dat de vermogensfluctuaties in het opslagsysteem zo groot zijn, dat een onaanvaardbare korte levensduur van de conventionele batterij te verwachten is. Hiervoor is een oplossing uitgewerkt om de batterij systemen te verlichten
met behulp van super capaciteiten, die ontworpen zijn voor snelle
vermogensuitwisselingen. Door deze componenten samen te laten werken, kan de levensduur van het batterij systeem aanzienlijk worden vergroot. Deze laatste analyse is met behulp van computersimulaties (Matlab) geanalyseerd en nog niet in hardware uitgevoerd,
Geconcludeerd kan worden dat een laboratorium voor duurzame energie is gerealiseerd, waarmee de autonome toepassing geanalyseerd kan worden met een schaalgrootte tot 10 huishoudens (50 kVA). Daarnaast is met enige aanpassingen ook de „zwak-netgekoppelde‟, de direct netgekoppelde en de parallel opererende situatie te creëren. De flexibiliteit waarmee het systeem is opgezet biedt tevens de mogelijkheid om ook met gedrag van andere opwekkers mee te nemen in de analyse, zowel hardwarematig als softwarematig.
Over het onderzoek naar autonome duurzame energie systemen kan gezegd worden dat de autonome bedrijfsvoering mogelijk is, maar dat het niet eenvoudig is voor een systeem op beperkte schaal. Daarnaast dat er een grote mate van over-dimensionering noodzakelijk is, vanwege de vele verliezen in het opslagsysteem én het voorkomen van energie tekorten gedurende bepaalde perioden van het jaar. Hoe een autonoom systeem zich manifesteert ten opzichte van andere alternatieven, kan niet op voorhand gezegd worden. De technische en economische ontwikkelingen zullen afgewacht moeten worden om te zien of in de (nabije) toekomst een autonoom systeem ook in Nederland rendabel zal blijken. Technisch gezien zijn er geen onoverkomelijke beperkingen waardoor het niet mogelijk zou zijn om de elektriciteitsvoorziening duurzaam én autonoom te organiseren. Arjan van Voorden
Samenvatting 10
List of figures 16
List of abbreviations 19
1 Introduction 20
1.1 Future power systems 20
1.2 Sustainability 24
1.3 The SDE - project 27
1.4 Renewable energy research at the DUT 28
1.4.1 DIOC-SENECU 28
1.4.2 Wind energy institute DUWind 29
1.4.3 Sustainable energy research in the faculty EEMCS 30
1.5 Outline of the thesis 31
2 Autonomous systems 32 2.1 Energy-neutral system 32 2.2 Stand-alone systems 33 2.2.1 Generator-control 33 2.2.2 Demand-control 35 2.2.3 Buffer control 37 3 Battery Storage 40
3.1 Working principal lead acid batteries 40
3.2 Determination State of Charge 43
3.2.1 The specific gravity 43
3.2.2 Terminal voltage measurement 44
3.2.3 Open terminal voltage 44
3.2.4 Ampere-hour method 47
3.3 Battery limitations in autonomous renewable operation 49
4 Energy calculations 52
4.1 Solar energy 52
4.1.1 PV in silicon: basic principles 52
4.1.2 Technology of solar cells 54
4.1.3 Irradiance and panel orientation 55
4.1.4 Potential in the Netherlands 59
4.1.5 Measurement data in the laboratory 62
4.2 Wind energy 63
4.2.1 Potential in the Netherlands 63
4.2.2 Wind data of KNMI 64
4.2.3 Wind energy in the built environment 66
4.2.4 Measurement data in the laboratory 69
4.2.5 Wind versus height 70
4.3 Combined Heat and Power 73
4.4 Power demand of households 75
4.5 Stochastic nature of the data 77
4.5.1 Hourly values 78
4.5.2 Daily energy values 79
4.6 Sizing the components 80
4.7 Economic aspects 83
4.7.1 Overview of the equipment costs 83
4.7.2 Life-cycle costs 84
4.8 Conclusions 86
5 Design of the renewable energy laboratory 87
5.1 Wind turbine simulator 87
5.1.1 General concept 88
5.1.2 Soft starter 92
5.2 CHP simulator 93
5.3 Battery simulator 93
5.3.1 Battery controller 96
5.4 Load demand simulator 96
5.5 PhotoVoltaic system 98
5.6 The complete DENlab system 101
5.6.1 Galvanic insulation and transformers 101
5.6.2 Monitoring and Control 102
5.6.3 Earthing and Protection 103
5.6.4 Electro Magnetic Compatibility 104
5.7 Conclusions 105
6 System control strategies 106
6.1 Operation bandwidth of the SoC 107
6.2 The application of two battery stacks in parallel 112
6.3 Load forecasting incorporated in the strategy 115
6.4 Equalization with the use of two battery arrays 118
6.5 Quantitative comparison 120
6.6 Conclusions 123
7 Results 124
7.1 Voltage stability during normal operation 124
7.2 Voltage stability during high power changes 126
7.3 Harmonic components in the bus voltage 127
7.4 Evolution of the State of Charge 128
7.5 Control actions by the Programmable Power Control Unit 130 7.5.1 Start and Stop of the Combined Heat and Power unit 130
7.5.2 Evaluating the operation of the WECS 131
7.5.3 Load demand simulator 132
7.5.4 System power exchange 133
8 Renewable Energy Forecasting 135
8.1 Wind Forecasts 135
8.1.1 Persistence 136
8.2 Sun Forecasts 141
8.3 Results with forecasting method 144
8.4 Conclusions applying forecasting 148
9 Super Capacitors 149
9.1 Theory 150
9.2 Different battery-supercapacitor combinations 152
9.2.1 Model simplifications 155
9.2.2 A comparison between the DC and AC-configuration 158
9.3 Sizing of SC systems 159
9.4 Control strategy 161
9.5 Simulation results 161
10 Conclusions 166
References 169
Appendix Benchmarking project for hybrid power systems 172
LIST OF FIGURES
Figure 1-1 Historical and predicted electricity demand in the Netherlands... 20
Figure 1-2 Driver forces to new Power Systems, source: SmartGrids [6] ... 23
Figure 1-3 the relationships between the different SDE-projects ... 28
Figure 2-1 AGC control action ... 35
Figure 2-2 Example of dump-load application in AES ... 36
Figure 2-3 Electrical appliances with different properties ... 36
Figure 2-4 A Three phase inverter ... 37
Figure 2-5 Sinusoidal PWM ... 38
Figure 2-6 Harmonic spectrum ... 39
Figure 3-1 State of Charge indices during charge and discharge operation ... 43
Figure 3-2 Charging voltage as a function of SOC and the C-rate ... 45
Figure 3-3 Discharge voltage vs. SOC and C-rate ... 45
Figure 3-4 Trend line (polynomial) ... 46
Figure 3-5 The P-factor as a function of the discharge currents ... 48
Figure 3-6 Charge efficiency near top-of-charge ... 49
Figure 3-7 A number of battery problems ... 50
Figure 4-1 PN-process by incoming photons ... 53
Figure 4-2 Solar radiation components ... 55
Figure 4-3 Irradiance diagram for the Netherlands ... 56
Figure 4-4 Orientation angles for the solar modules ... 57
Figure 4-5 Solar panels output power from direct and diffuse radiation ... 58
Figure 4-6 Total hourly solar power (Psolar) pattern for 1996 & 1997 ... 59
Figure 4-7 Monthly global irradiance pattern 1996 ... 59
Figure 4-8 Monthly global irradiance pattern 1997 ... 59
Figure 4-9 Global sun irradiations in the Netherlands in kWh/year ... 60
Figure 4-10 PV power on July 6. and July 7. in 2004, measured in DENLAB (120m2 ) ... 61
Figure 4-11 PV output power, based on measurements in DENlab 2005 ... 62
Figure 4-12 Average wind speed in the Netherlands ... 64
Figure 4-13 Distribution and Cumulative frequency distribution of wind speed at Zestienhoven ... 65
Figure 4-14 Wind rose calculated from station Zestienhoven ... 66
Figure 4-15 CFD calculations of wind turbulence around a building ... 67
Figure 4-16 Zone of wind turbulence caused by an obstacle ... 68
Figure 4-17 Satellite view of the DENlab anemometer site ... 68
Figure 4-18 One minute average of the wind speed on 1-5-2004, measured in DENlab ... 69
Figure 4-19 Power Curve of the Turby (left) and Furlander FL30 (right) ... 71
Figure 4-20 Fuhrlander FL30 (left) and Turby (right) ... 71
Figure 4-21 Calculated values of wind turbine output in DENlab ... 72
Figure 4-22 Schematic of micro CHP ... 74
Figure 4-24 Typical load pattern for different number of houses... 76
Figure 4-25 Random four weeks load pattern of 10 households ... 77
Figure 4-26 Scatter plot with hourly values of PV and Wind ... 78
Figure 4-27 Scatter plot with daily values of PV and Wind ... 79
Figure 4-28 Daily energy of solar and Load ... 80
Figure 4-29 Effect of battery storage on auxiliary power contribution ... 81
Figure 4-30 Effect of battery storage of hours of operating aux. source ... 82
Figure 5-1 Set-up of the wind turbine simulator ... 88
Figure 5-2 Example of the power curve for a stall-regulated wind turbine ... 89
Figure 5-3 Motor-generator set 37 kW as it is installed in DENlab ... 89
Figure 5-4 Full torque-speed characteristic for three different stator frequencies ... 90
Figure 5-5 Soft starter circuit ... 92
Figure 5-6 Example of soft starter characteristic ... 92
Figure 5-7 Motor-generator set representing the CHP (left Motor, right Generator) ... 93
Figure 5-8 Circuit diagram of the battery simulator ... 94
Figure 5-9 An AC/DC/AC system representing the battery simulator... 95
Figure 5-10 A feedback system to control the battery current ... 96
Figure 5-11 Load demand simulator ... 97
Figure 5-12 Inverter part of the load demand simulator ... 97
Figure 5-13 Roof for PV-panels (1st part), at the right-side the high-rise building ... 98
Figure 5-14 Detail of PV installation ... 99
Figure 5-15 Set-up of PV-panels at DUT ... 99
Figure 5-16 View of the first part of the PV system ... 100
Figure 5-17 Actual set-up of the DENlab laboratory ... 101
Figure 5-18 User interface of the laboratory system ... 103
Figure 5-19 TN-S system ... 103
Figure 5-20 Snapshot of the laboratory system ... 104
Figure 6-1 State diagram first strategy ... 107
Figure 6-2 Flowchart first strategy ... 108
Figure 6-3 Plot of results for entire year (first strategy) ... 109
Figure 6-4 Plot of results for first month (first strategy) ... 109
Figure 6-5 Simulation result with excessive power variant ... 112
Figure 6-6 State diagram second strategy ... 113
Figure 6-7 Plot of results for the entire year (second control strategy) ... 113
Figure 6-8 Plot of results for first month (second control strategy) ... 114
Figure 6-9 Control diagram (third control strategy) ... 115
Figure 6-10 Plot of results for entire year (third control method) ... 116
Figure 6-11 Plot of results for first month (third control method) ... 116
Figure 6-12 Control diagram (fourth control strategy) ... 118
Figure 6-13 Plots of the results for entire year (fourth control strategy) ... 119
Figure 6-14 Plot of the result of the first month (fourth control strategy) ... 119
Figure 7-1 Basic circuit Active Frond End Converter ... 125
Figure 7-3 Measurement of the voltage of the autonomous grid... 126
Figure 7-4 Voltage waveform at the autonomous rail ... 127
Figure 7-5 Harmonic Spectrum of the voltage waveform ... 127
Figure 7-6 Battery current and SoC evolution ... 128
Figure 7-7 CHP start at low SoC level... 130
Figure 7-8 Wind speed and power of the WECS ... 131
Figure 7-9 Load demand pattern... 132
Figure 7-10 Measurement result of DENlab system ... 133
Figure 8-1 RMS errors for different forecasting models ... 135
Figure 8-2 Autocorrelation values for wind speed and power time series, for DENlab and S344 ... 137
Figure 8-3 Wind forecasting errors for different ... 138
Figure 8-4 Wind power forecasts compared with the real wind power produced ... 140
Figure 8-5 a) models for sun power forecasting b) Autocorrelation function for sun power output in 2005 ... 141
Figure 8-6 Simple persistence (p) and new persistence (np) results for sun forecasting ... 142
Figure 8-7 MAE and RMSE for six models predicting sun power for 2005 ... 143
Figure 8-8 Real solar output vs. one and three hours forecast ... 143
Figure 8-9 General structure of simulation programs ... 145
Figure 8-10 Cost function values for 2005 models, with the three different types of forecasts ... 147
Figure 9-1 Overview storage systems source: electricitystorage.org ... 149
Figure 9-2 Construction of a Double Layer Capacitor... 150
Figure 9-3 S.C. price evolution from 1996 to 2010 (expected), source: Maxwell (2006) ... 151
Figure 9-4 Super capacitor and Battery combination on one DC-system ... 152
Figure 9-5 Matlab Simulink model of option B ... 153
Figure 9-6 Currents [Amps] in DC- Super capacitor and Battery combination ... 154
Figure 9-7 Voltages [V] of Super capacitor and Battery combination in DC-system ... 154
Figure 9-8 Supercapacitor-battery system with decoupled DC-circuits ... 155
Figure 9-9 Simplified models for VSC and CSC converters ... 156
Figure 9-10 Powers vs. time for decoupled DC-buses ... 157
Figure 9-11 Super-capacitor battery system with decoupled DC-buses ... 158
Figure 9-12 Power exchange between SC and battery ... 160
Figure 9-13 Battery current control system ... 161
Figure 9-14 Final autonomous energy system, with an integrated SC storage system ... 162
Figure 9-15 Power exchange between system components during 1 hour ... 163
Figure 9-16 Power exchange between system components during 2 minutes ... 164
AGM Absorbent Glass Mat
ARES Autonomous Renewable Energy System
CHP Combined Heat and Power
DG Distributed Generation
DoD Depth of Discharge
DSM Demand Side Management
FET Field Effect transistor
KNMI Royal Dutch Meteorological Institute
LCC Life-cycling costing
PCC Point of Common Coupling
PLC Programmable Logic Controller
PV Photo-Voltaic
PWM Puls-Width Modulation
RES Renewable Energy Source
SC Super Capacitor
SDE Samenwerkingsverband Duurzame Energy
SoC State of Charge
UPS Uninterruptible Power Supply
VRLA Valve-Regulated Lead Acid (battery)
The history of the electrical engineering started with the early experiments of Benjamin Franklin (1706-1790), Michael Faraday (1791-1867), Luigi Galvani (1737-1798), Alessandro Volta (1745-1827), André-Marie Ampère (1775-1836) and Georg Simon Ohm (1789-1854). Because of their inventions, there became a fundament for an electrical society. The influence of the electricity in the modern society is nowadays essential: a world without electricity is now inconceivable. In particular during power failures, i.e. black-outs, the society‟s dependency on electrical energy becomes perceptible. A high quality design of the electrical infrastructure, with redundancy, the continuous monitoring of the system and maintenance of the system components has resulted in a very reliable power system in particular in the economically strong countries. The Dutch power system is currently one of the most reliable systems in the world.
In the last decade, the electrical energy consumption has grown in the Netherlands from 80 TWh in 1990 to 108 TWh in 2000, being an increase of 35%. Estimations show that the electricity consumption in 2010 will be 125 TWh [1].
Figure 1-1 Historical and predicted electricity demand in the Netherlands
1.1 Future power systems
Traditional Power Systems are nowadays changing under the influence of two mechanisms: the market operation of the power system and the introduction of renewable energy conversion systems. [2-6].
The electrical infrastructure in most OECD Countries (Organisation for Economic Co-operation and Development) is typically composed of rather large power systems with ageing generation and grid assets. In 2010, close to approximately 65% of this central power generation will be based on fossil fuels. In order to break this trend and increase generation efficiency a substantial replacement and refurbishment program will have to be initiated within the next years. The historical development of transmission and distribution assets is closely associated with the development of generation but the technical lifetime is often longer.
Another important driver is the increasingly strong focus being placed on environmental issues. This encompasses generation as well as transmission and distribution. In the field of generation, environmental concerns drive the search for cleaner technologies (less or no greenhouse emission) as well as higher energy conversion efficiencies. In transmission and distribution area, a reduction of visual, electromagnetic and acoustic impact is foreseen as well as a reduction in transmission losses.
Yet another driver is the liberalisation of energy markets currently taking place, which influences the properties of the markets. I.e. the investments have been kept relatively low for the past decade due to the competitive pressure. Regulators and investors therefore actively seek new mechanisms for new technical solutions in order to cope adequately with the electricity market dynamics.
The electricity sector faces new challenges and opportunities which must be responded to in a vision of the future [6]:
User-centric approach: increased interest in electricity markets opportunities, value added services, flexible demand for energy, lower prices, micro-generation opportunities;
Electricity networks renewal and innovation: pursuing efficient asset management, increasing the degree of automation for better quality of service; using system wide remote control; applying efficient investments to solve infrastructure ageing;
Security of supply: limited primary resources of traditional energy sources, flexible storage; need for higher reliability and quality; increasing network and generation capacity;
Liberalised markets: responding to the requirements and opportunities of liberalisation by developing and enabling both new products and new services; high demand flexibility and controlled price volatility, flexible and predictable tariffs; liquid markets for trading of energy and grid services;
Interoperability of European electricity networks: supporting the implementation of the internal market; efficient management of cross border and transit network congestion; improving the long-distance transport and integration of renewable energy sources; strengthening European security of supply through enhanced transfer capabilities;
Distributed generation (DG) and renewable energy sources (RES): local energy management, losses and emissions reduction, integration within power networks;
Central generation: renewal of the existing power-plants, development of efficiency improvements, increased flexibility towards the system services, integration with RES and DG;
Environmental issues: reaching Kyoto Protocol targets; evaluate their impact on the electricity transits in Europe; reduce losses; increasing social responsibility
and sustainability; optimising visual impact and land-use; reduce permission times for new infrastructure;
Demand response and demand side management (DSM): developing strategies for local demand modulation and load control by electronic metering and automatic meter management systems;
Politics and regulatory aspects: continuing development and harmonisation of policies and regulatory frameworks in the European Union (EU) context;
Social and demographic aspects: considering changed demand of an ageing society with increased comfort and quality of life.
Today‟s electricity networks provide an essential service for society, built to ensure access for every single electricity customer. The power flow was controlled by a vertically operated system with dominant power plants, a distributed consumption and no direct interconnection between the control areas.
In response to new challenges and opportunities, electricity networks have started to evolve. The aim is that they accommodate more decentralised generation services, with many actors involved in the generation, transmission, distribution and operation of the system.
Future visions on Power Systems differ in details. The CIGRE Workgroup JAG 02 TC [2] supposes that a centralised generation structure, based on the present power system, prevails. Two scenarios are proposed: the distribution scenario and the „mixtribution‟ scenario. The last scenario is closely related to the SMARTGRIDS vision [6], which predicts that Europe‟s electricity networks in 2020 and beyond will be “FARE”:
Flexible Fulfilling customers‟ needs whilst responding to the changes and
challenges ahead;
Accessible Granting connection access to all network users, particularly for renewable energy systems (RES) and high efficiency local generation with zero or low carbon emissions;
Reliable Assuring and improving security and quality of supply, consistent
with the demands of the digital age;
Economic Providing best value through innovation, efficient energy
management and „level playing field‟ competition and regulation.
Driving factors in the move towards SmartGrids are the European Internal Markets, the security and quality of supply and the environment.
Liberalisation
Low prices and efficiency Innovation and competitiveness Primary energy availability Capacity
Reliability and quality
Nature and wildlife preservation
Climate change Pollution
INTERNAL MARKET
ENVIRONMENT
SECURITY OF SUPPLY
Figure 1-2 Driver forces to new Power Systems, source: SmartGrids [6]
Hydro- and nuclear power plants are well established methods of generation with nearly zero greenhouse gasses. Accommodating change may be possible by incorporating new generation technologies. Successful examples are wind turbines and solar panels, but there are other distributed generation technologies that are either already commercial or close to being on the market, such as micro combined heat and power systems (CHP). These forms of power generation have different characteristics from traditional power plants. Most of them have much smaller electricity outputs; some of the newer technologies also exhibit greater intermittency.
It is difficult to predict the impact of distributed generation on the future energy mix. Research in this area is essential.
There is uncertainty in many aspects of future grids: In the primary energy mix;
In the electricity flows created by the liberalised market;
Of the instantaneous power output of many renewable energy sources; In regulatory frameworks and investment remuneration in innovation.
The best strategy for managing these uncertainties is to build flexibility and robustness into the future network.
Distribution grids will become active and will have to accommodate bi-directional power flows. The European electricity systems have moved to operate under the framework of a market model.
The future grids will have six properties:
User specified quality, security and reliability of supply for the digital age; Flexible, optimal and strategic grid expansion, maintenance and operation; Flexible DSM and customer-driven value added services;
Harmonised legal frameworks facilitating cross-border trading of power and grid services;
Massive small, distributed generation connected close to end customers; Coordinated, local energy management and full integration of DG and RES. This study will focus on the last two topics.
1.2 Sustainability
Sustainable development is an important issue for the coming century [4]. The new built areas in 2020 will already be influenced by the drive for sustainability. An often-quoted definition of sustainability, proposed by the commission Brundlandt, is [7]:"Meeting the
needs of the present generation without compromising the needs of future generations to meet their own needs."
It includes two key concepts:
The concept of „needs‟, in particular the essential needs of the world‟s poor part that overriding priority should be given; and
The idea of limitations imposed by the state of technology and social organization on the environment‟s ability to meet present and future needs.
Interpretations will vary, but share certain general features. It must develop from a consensus on the basic concept of sustainable development to a broad strategic framework for achieving it. The development involves a progressive transformation of economy and society. A development path that is sustainable in a physical sense could theoretically be pursued.
Sustainable development authors view the challenge of sustainable consumption as a spectrum. At the near end of this spectrum are measures that require less in terms of intervention and active change. Examples of simple technological interventions are the installation of solar cells and a big cut in standby power requirements for audio and video equipment. They could have a very positive environmental impact. In the centre of the spectrum are more radical changes in habits and routines, such as restoring a sense of seasonality to what we eat, turning off lights and opting to walk or cycle in the neighbourhood rather then taking the car. At the far end of the spectrum are innovations and measures that allow people to change behaviour in a more fundamental way, such as limiting the air traffic. [8]
Meeting essential needs partly depend on achieving full growth potential and sustainable development clearly requires economic growth in places where such needs are not being met.
A society may in many ways compromise its ability to meet the essential needs of future generations by overexploiting resources, for example. The direction of technological developments may solve some immediate problems but may lead to even greater ones in the future.
As for non-renewable resources, like fossil fuels and minerals, their use reduces the stock available for future generations. But this does not mean that such resources should
not be used. In general the rate of depletion should take into account the criticality of that resource, the availability of technologies for minimizing depletion, and the likelihood of substitutes being available. Sustainable development requires that the rate of depletion of non renewable resources should excluded future options as few as possible.
In essence, sustainable development is a process of change in which the exploitation of resources, the direction of investments, the orientation of technological development and institutional change are all in harmony and enhance both current and future potential to meet human. As the commission Brundlandt says: The world must quickly design strategies that will allow nations to move from present, often destructive, processes of growth and development onto sustainable development paths.
Energy is an essential human need, one that cannot be universally met unless energy consumption patterns change. The ultimate limits to global development are perhaps determined by the availability of energy resources and by the biosphere‟s capacity to absorb the by-products of energy use. First, there are the supply problems: the depletion of oil reserves, the high cost and environmental impact of coal mining and gas winning, and the hazards of nuclear technology. Second, there are emission problems, most notably acid pollution and carbon dioxide build-up leading to global warming.
Some of these problems can be met by increasing use of renewable energy sources. But the exploitation of some renewable sources such as biomass and hydropower also entails ecological problems. Hence sustainability requires a clear focus on conserving and efficiently using energy.
In its broadest sense, the strategy for sustainable development aims to promote harmony among human beings and between humanity and nature. In the specific context of the development and environmental crises of the 1980s, which current national and international political and economic institutions have not and perhaps cannot overcome, the pursuit of sustainable development requires:
A political system that secures effective citizen participation in decision making; An economic system that is able to generate surpluses and technical knowledge
on a self-reliant and sustained basis;
A social systems that provides solutions for the tensions arising from disharmonious development;
A production system that respects the obligation to preserve the ecological base for development;
A technological system that can search continuously for new solutions; An international system that fosters sustainable patterns of trade and finance; An administrative system that is flexible and has the capacity for self-correction. These requirements are more in the nature of goals that should underlie national and international action on development.
Focusing on energy issues, it‟s clear that today‟s primary sources of energy are mainly non-renewable: natural gas, oil, coal and conventional nuclear power. There are also renewable sources, hydro power, geothermal sources, solar, wind, and wave energy, as well as biomass. In theory, all the various energy sources can contribute to the future energy mix, but each had its own economic, health and environmental costs, limitations, benefits and risks. Choices must be made, but in the certain knowledge that choosing an energy strategy inevitably means choosing an environmental strategy.
Patterns and changes in energy usage today are already dictating patterns well into the next century. According to the Brundlandt commission, the key elements of sustainability that have to be reconciled are:
Sufficient growth of energy supplies to meet human needs;
Energy efficiency and conservation measures, such that waste of primary resources is minimized;
Public health, recognizing the problems of risks to safety inherent in energy sources;
Protection of the biosphere and prevention of more localized forms of pollution. The crucial point about lower energy-efficient is not whether they are perfectly realizable in their proposed time frames. Political and institutional mind shifts are required to restructure investment potential in order to move along these lower consumption or more energy-efficient paths.
Many forecasts of exploitable oil reserves and resources suggest that oil production will level off by the early decades of the next century and then gradually fall during a period of reduced supplies and higher prices. Gas supplies should last over 200 years and coal about 3000 years at present rates of use. The estimates persuade many analysts that the world should immediately embark on a vigorous oil conservation policy. In terms of pollution risks, gas is by far the cleanest fuel, with oil next and coal a poor third. But they all pose three interrelated atmospheric pollution problems: global warming, urban industrial air pollution and acidification of the environment.
Focussing on the built-up environment, a broad range of technical measures for improving energy efficiency in homes are already well understood to be cost-effective and readily available on the market. These are: insulation, draught proofing, secondary and double glazing, improved heating systems, wider use of heating controls and efficient lighting and appliances. Installing micro-generation also reduces demand for centrally supplied energy, such as with solar hot water systems, ground source heat pumps and photovoltaics. Micro CHP boilers allow households to generate electricity whilst consuming gas for heating, and community wide CHP schemes provide probably the most cost effective option for reducing demand for energy in certain housing densities. To encourage more sustained and widespread adoption of energy efficiency measures, the steps in the following directions can be taken: [9]
Improved consumer information through measures such as installing smart meters into houses, liked on the fuel bills or compared to typical households; Improved incentives and requirements for building contractors and energy
suppliers in homes or the introduction of personal carbon credit system.
Renewable energy sources could in theory provide the global energy consumption [10]. Solar energy use is small globally, but it is beginning to assume an important place in the energy consumption patterns of some countries, especially in the „hot countries‟ like Australia, Greece and the Middle East.
Wind power has been used for centuries – mainly for pumping water. Recently its use has been growing rapidly in regions such as California and Scandinavia. In these cases, the wind turbines are used to generate electricity for the local grid. Many countries have already successful programs, but the untapped potential is still high.
Most renewable energy systems operate best at small to medium scales, ideally suited for rural and suburban applications. The need for a steady transition to a broader and
more sustainable mix of energy sources is beginning to become accepted. Renewable energy sources could contribute substantially to this.
1.3 The SDE - project
In 1997 a third-partner project was initiated to design an autonomous energy system for supplying energy to households. The project focuses on the development of a Power Control Unit that supervises and controls an energy system, powered by renewable energy sources that supplies approximately 10 households. In the design, a small wind turbine, solar panels and a micro Combined Heat and Power system (-CHP) were selected to supply the necessary electrical energy for ten households. To balance the generated and consumed power a battery storage system was foreseen. The design of the laboratory environment to carry out this project is the main subject of this initial project.
Preliminary calculations were made, mainly based on simplified models of the generation system as such in combination with mean values of wind speeds, solar irradiations and load side power demands. This research resulted in a general impression about the feasibility of an autonomous renewable energy system in relation to the local conditions. This rough impression was, however, not sufficient to develop the Power Control Unit and the available computer models did not suit for the power range of this particular case. Apart from that, experience with other projects has learned that building a system in practice is in fact the best way to study and understand the system.
In 1999, we decided to the build the DENlab ® laboratory model for the study of autonomous system with three aims:
To develop a Programmable Power Control Unit; To model system components with practical verification;
To acquire practical knowledge about how to set up autonomous renewable energy systems;
Based on a project application, SDE (Samenwerkingsverband Duurzame Energie), a cooperative body initiated by the Ministry of Economic Affairs chooses this project to participate in its research program: System Integration of Renewable Energy in the built-environment.
The participating partners in this research program are KEMA-Arnhem, IVAM-Amsterdam, ECN-IVAM-Amsterdam, ECN-Petten, TNO-MEP Apeldoorn and the Delft University of Technology (DUT).
The following topics were addressed in this research:
The prediction of contribution of wind and solar energy to the energy balance for the coming 24 to 48 hours; (ECN-Petten)
The generation of a realistic load demand pattern generator based on the input of questionnaires as well as on actual measurements; (KEMA-IVAM)
To find out how load demand can influence the user behaviour by setting goals and giving feedback (ECN-Amsterdam)
To develop of a functional description of an overall power controller (KEMA) To develop and to test a prototype of an optimised power control unit for
User
Behaviour and Acception
Energy demand
Pattern and Prediction Household demand
System
Control Optimization
Sources
Pattern and Predictions Solar and Wind
Functional design
Controls and Protection Test facility and PPCU Monitoring and Evaluation
Tested Prototype
Figure 1-3 the relationships between the different SDE-projects
1.4 Renewable energy research at the DUT
The technological demands arising from the challenges facing society are becoming increasingly complex. Protecting the land from the water, for example, is not simply a question of building dikes. Other factors have to be taken into consideration, including the minimum acceptable human and capital risks, climate change and changing societal needs. These complex problems need to be studied from a multidisciplinary perspective when we want to find robust and sustainable solutions. The thirteen Delft Research Centres (DRCs) provide integrated solutions for these multidisciplinary problems by bundling excellent research. In this way, the knowledge and expertise built up over the years on thirteen important research themes is made available to society, industry and policy. This thesis is partly a result from research in the framework of one of these thirteen DRCs: Sustainable Energy.
1.4.1 DIOC-SENECU
The Delft University of Technology decided in 1997 to direct a substantial part of the University's fundamental and applied research to a strong contribution to the research and development of cleaner energy systems, which obviously do include sustainable energy sources. They are the targeted fields of research and development for securing our future energy supply and, in particular, protecting the environment. Hence, the scientific research program of the Technology Theme Sustainable Energy: Extraction, Conversion and Use has the ambition to deliver important contributions to the utilization
of sustainable energy via in depth fundamental and applied research and development projects. In line with the recent interest on a National Level in Sustainable Hydrogen, research on sustainable energy conversion and storage of hydrogen constitutes an important part of the present Technology Theme.
The scientific program of the Technology Theme "Sustainable Energy: Extraction, Conversion and Use" is based on the sub - themes: Solar Energy Conversion, Wind Energy in the Built Environment, Biomass Gasification and Gas Purification, Storage of Electrical Energy in Rechargeable Batteries, Production and Storage of Sustainable Hydrogen, Conversion of Chemical to Electrical Energy using Fuel Cells, and Power Electronics. Attention will also be focused on the societal acceptance of sustainable energy sources, in particular, of hydrogen.
The Technology Theme has its roots in the program "Decentralized Production and Storage of Electricity for Large-Scale Application of Renewable Energy" of the Delft Interfaculty Research Centre (DIOC) "Extraction, Conversion and Use of Energy", which was executed very successfully in the period 1997-2001, and the more recent program "Sustainable Hydrogen: Energy Carrier of the Future", which received financial support from TU Delft in December 2001. The continuation of the first program and the "Sustainable Hydrogen" program constitute the program of the present Technology Theme. It is emphasized that as a prelude to a Hydrogen Economy, the present Technology Theme comprises a substantial number of projects directed to the production of hydrogen, using sustainable energy sources, the safe and cheap storage of hydrogen, supported by fundamental characterization studies, and utilization.
The search for cleaner, cheaper, and more efficient alternative energy technologies has been driven by recent developments in materials science and engineering. Indeed, advanced solar cells with nanostructures, rechargeable batteries based on materials with nanostructures, hydrogen storage composite materials, based on light-weight metal hydrides and transition-metal oxide catalysts, IT-SOFC components, and wind turbine blades are and will be to a large extend, the result of materials science and engineering successes.
Biomass gasification and gas purification is another route to sustainable hydrogen and is supported by advanced reactor technology. The use of high-temperature waste heat for the thermo chemical production of hydrogen is an important feasibility study.
At the Delft University of Technology unique experimental characterization and theoretical modelling techniques are available. These will guide the materials scientists in designing, in particular, storage devices for electrons and hydrogen.
1.4.2 Wind energy institute DUWind
The wind institute, DUWind, has founded in August 1999 to stimulate the synergy between the different disciplines which are related to implementation of wind energy. The wind institute covers the wide scope of topics as erecting and founding wind turbines, its aero-dynamic aspects, its off-shore aspects and electro dynamical behaviour of wind turbines. Each discipline of the participating research groups has its own specific expertise, but an increasing number of research problems require a multi-disciplinary approach.
On all these issues the Section Wind Energy has contributed significantly to the increase of knowledge and the development of numerical tools. The SWING® series of numerical
wind field simulators, the DU-W® airfoil family, the airfoil design code XFOIL, the rotor
aerodynamic codes PREDICHAT and PREDICDYN, and the DUWECS® dynamic wind
turbine simulator have been results of these modelling efforts.
Since the beginning of the nineties offshore application of wind turbines is also a field of research interest at Delft University. The dynamic loads code DUWECS® was extended to incorporate wave loads, and a more extensive soil model was added to incorporate the effects of variation of soil properties. The latter is of special importance for the modelling of monopod foundations. Over the decade an integrated design approach was developed for large scale offshore wind farms. Apart from integrated numerical structural models with simultaneous wind and wave loads, such an approach required the development of cost models and operational research models. The integrated design philosophy was first demonstrated in the design solution presented within the Opti-OWECS project, an international EU funded research project, which was coordinated by the TU Delft.
The use of wind energy in the built environment has gained new interest. Apart from the application of standard "propeller-type" wind turbines near buildings the research focuses on the potential of specially adapted wind turbines, often with a vertical axis and slow running, and of "passive" use of wind power. In the latter case it is tried to use pressure differences originating from ambient wind conditions to perform work, e.g. by powering natural ventilation or by augmenting the performance of more or less standard wind turbines on top of a building.
1.4.3 Sustainable energy research in the faculty EEMCS
Within the faculty Electrical Engineering, Mathematics and Computer Science (EEMCS), the sustainability projects are mainly concentrated in the Electrical Power Engineering (EPE) department. Only one research group outside the EPE department has activities on the renewable energy field; the microelectronic department (DIMES) investigates the development of new solar cells techniques.
This EPE department consists of the High Voltage Technology and Management laboratory (HTM), the Electrical Power Processing laboratory (EPP) and the Electrical Power Systems laboratory (EPS).
The sustainable research issues of HTM group are concentrated on the research on materials. Some of the High-Voltage switchgear uses the toxic and environmental unfriendly insulation gas SF6 and polluting oil. Research is essential to investigate the applicability and the properties of other materials. On the other hand, life time extension of the Power System assets is investigated in order to minimize or at least extend the need of replacement of the components.
The EPP-group is a research group focusing on energy conversion issues: power electronics and electrical machines and drives. They made a substantial contribution in renewable energy solutions such as the Archimedes Wave Swing (applying wave energy as a renewable power source) and new types of generators for wind turbines. For the application and conversion of renewable energy sources, the power electronics is more and more important. Within the framework of the EMVT-IOP (A Dutch acronym for Electrical Magnetic Power Technology Innovative Research Program), research is carried out to develop power electronic solutions in order to make the future power system intelligent.
